1. Field of the Invention
The present invention relates to a cylinder block for a hydraulic or hydrostatic piston device, but only the term hydraulic will be used hereinafter for convenience. More particularly, this invention relates to a hydraulic axial piston device having a bronze control surface.
2. Description of Prior Art
One type of hydraulic axial piston device known in the prior art has a steel cylinder block and a control layer formed by casting a bronze layer onto the end surface of the cylinder block (also called a cylinder drum). The bronze layer provides the sliding friction characteristics at the surface of the control layer adjacent to the valve mechanism. Also, each cylinder bore has a bronze or brass bearing bushing for sliding reciprocating movement of the pistons. The cylinder bore in such a cylinder block has, therefore, a greater diameter than the pistons by virtue of the thickness of the brass bushings. If those brass bushings could be eliminated and the cylinder block provided with a piston mounted for reciprocation directly against the inner walls of the cylinder bore, the pistons and thus the power stroke volume would be greater. Alternatively, a stronger and more durable cylinder block can be created if the brass bushings could be eliminated and the thickness of the cylinder block between the cylinders and between the cylinders and the outer wall of the cylinder block could be increased. Also, elimination of the brass bushings in the cylinder block would reduce any tendency of pistons sticking within the brass bushings because of the different rates of heat expansion between material of the piston and the brass bushings and the tendency of the inner diameter of the bushing to be reduced relative to the piston.
The well known practice of casting a bronze layer for the control surface of a cylinder block for a hydraulic axial piston device is disclosed in West German patent application No. 1,963,769. This reference also discloses that the cylinder block can be made from steel or nodular cast iron containing graphite in small spheres and that the inner walls of the cylinder bores can be hardened by liquid nitriding in order to slidably receive hardened steel pistons. However, the requirements for the process of casting the bronze control layer limits the choice of material for the cylinder block. Only a low alloyed steel or nodular cast iron can be utilized which further limits from a practical standpoint the choice of any additional heat treatment to liquid nitriding. Thus, the process parameters of this reference prevent the selection of a material which would lead to a more durable cylinder block.
Sintering a friction metal control layer onto the end surface of a steel cylinder block with the simultaneous action of heat and pressure is disclosed in West German patent application No. 24 31 254. Bronze bushings within each cylinder bore were utilized in this reference. It was not appreciated in this reference that gas nitriding the steel cylinder block would eliminate the need for the bronze bushings without affecting the properties of the sintered bronze layer. This reference includes the use of an intermediate electrodeposited copper layer between the cylinder block and the sintered layer.
Axial position devices with piston rods lying directly against the cylinder wall are disclosed in French patent application No. 73.29921. The drive flange rotates with the cylinder block. The piston rods transmit rotary motion by direct contact with the cylinder walls; the contact force is considerable, especially during changes of angular speed. At the same time, the piston rod slides on the cylinder wall in the direction of the piston movement and simultaneously rotates with sliding contact in the circumferential direction of the cylinder wall. Because of these stresses on the cylinder wall, it has been difficult to produce a cylinder block both strong enough to sufficiently resist these stresses and yet provide the desired frictional characteristics.
Copies of the foregoing references were appended to the original application papers.